No single development in the history of luminaire intensity control for the performing arts has had the impact of the silicon controlled rectifier in the form of the phase control dimmer. Such dimmers made true low voltage remote control of lamp loads practical, and with it, the mastering, presetting, and memory capabilities at the heart of modern control systems. The impact of the solid state dimmer extended beyond performance lighting to embrace the control of architectural lighting as well.
However, in the quarter century since the introduction of the solid state electronic dimmer and despite considerable variation in the design of the triggering circuit, the power stage of such dimmers has remained unchanged.
The power stage of all professional electronic dimmers consists of a pair of silicon controlled rectifiers (or a triac) and a magnetic inductor or "choke".
Refer now to FIG. 1 where the output half-cycle waveform typical of such a prior art power stage at approximately half intensity is illustrated. Line 101 indicates the AC input waveform. Line 102 illustrates the response of the thyristor itself. The thyristor turns on in approximately one microsecond, producing a step voltage transition equal to the instantaneous line voltage (and, in the case of triggering 90.degree. after the zero crossing, producing a step voltage transition with a value equal to the peak voltage of the AC line). This voltage step produces a large current step in the attached load. The combined effect is a burst of electromagnetic energy rich in harmonics from audio frequencies all the way into the commercial radio frequencies. The low frequency component of the EMI noise spectrum propagates through the power wiring towards the load, while the high frequency components propagate towards both supply and load as well as radiating from the conductors themselves. The resulting electromagnetic interference has a variety of undesirable effects on other electronic equipment, notably sound amplification, radio, and video equipment, the precise nature and severity of those effects determined by a complex mix of variables.
This pulse also produces a magnetostrictive contraction of the connected lamp filament inducing vibration which causes audible noise and decreases lamp life.
For this reason, the power stage of prior art electronic dimmers incorporates a magnetic inductor or "choke" to reduce the rate of current change, increasing the voltage rise time. Increasing rise time decreases both the total EMI power generated and attenuates the higher frequencies.
For any given choke design it is axiomatic that the longer the rise time and the better the EMI suppression the larger and heavier (and hence more expensive) the choke. Dashed line 103 indicates the effect of adding the inductor to the circuit. Shaded area 104 represents energy stored in the choke not all of which is returned to the circuit.
However, as a choke controls only the rate of current change, the voltage rise and hence noise spectrum is highly load dependent.
Further, as the choke does not affect the rate of thyristor turn on, there is still a step voltage transition present in the conductor between the thyristor and choke. Although this transition does not reach the load, it results in radiated interference, particularly if the choke has been mounted remotely from the thyristor to ease mechanical design problems.
Finally, an ideal choke would provide increasing attenuation with frequency. However, the chokes employed in most prior art dimmer designs have, due to cost considerations, had fairly high shunt capacitance and/or types of core materials which result in significantly less than ideal high frequency attenuation.
In sum, because the addition of a choke to prior art dimmer power stages has no effect on the step voltage transition of the power device, an EMI noise spectrum far greater than that predicted from the circuit's rise time will result. Depending upon the quality, some portion of the low frequency component will also reach the load. High frequency components will propagate towards the supply, and radiate from the conductor between the power device and the choke.
Due to several trends in dimmer application and in component costs, an electronic dimmer power stage producing modest EMI without the need for a large inductor has become highly desirable.
One such trend is the steady increase in the cost of chokes as a result of rising raw material and labor costs (in contrast to a decrease in the cost of a dimmer's active components). Because similar inductors are seldom employed by industry at large, no significant improvements in cost or performance can be expected as a result of research and development or of high volume production for other applications.
Another trend which has had major impact on dimmer economics is the recent transition from the construction of dimming systems designed around load-patching to a relatively modest number of large wattage dimmers to the "dimmer-per-circuit" approach which requires hundreds of smaller wattage dimmers.
Because dimmers, large or small, require a similar basic complement of electronic and mechanical components, dimmer-per-circuit systems require new economies in dimmer design if the size and cost of the total system is to be competitive with conventional systems based on a far smaller number of larger wattage dimmers. Centralized triggering circuitry has been proposed as a method of reducing costs, but it is the choke which constitutes the largest single component in a small dimmer and one of the most expensive.
Chokes have a variety of other, undesirable effects on prior art dimmer designs. Due to their weight, chokes require substantial mounting provisions, increasing a dimmer's mechanical costs. In fact, the chokes alone account for more than 50% of the total weight of many dimmer enclosures and add hundreds of pounds to the shipping weight of a portable lighting system.
Chokes waste electrical energy in the form of heat as a result of both I.sup.2 R and core losses (losses added to the voltage drop and switching losses across the devices). This "choke loss" reduces voltage to the lamp, which, at full conduction, results in a significant loss of maximum fixture intensity. This voltage drop is, however, non-linear with respect to output voltage so that deliberate distortion must be introduced into the control input/output voltage response (or "curve") of open-loop digital and analog dimmers to compensate. Voltage drop also varies with significant changes in dimmer load.
The use of a larger choke to increase rise time will also result in increased power dissipation in the choke, and therefore improvements in rise time come at the cost of heat increased heat generation. This heat generation restricts the density of dimmer packaging; affects associated wiring and components; and requires airflow for cooling. However, unlike semiconductor packages, the efficiency of choke heat transfer to the ambient cannot be significantly improved with techniques like heatsinking in order to increase packaging density and/or reduce operating temperatures.
Chokes also frequently suffer from magnetically-induced vibration at certain phase angles, which, without careful isolation, produces audible noise which can distract the audience or be detected by sound recording equipment. This isolation, of course, is often at odds with the requirement for proper cooling.
Certain choke designs can also generate strong magnetic fields which have undesirable effects on nearby electronic equipment.
The recent trend to dimmer-per-circuit construction for portable dimming systems with its intrinsic requirement for a large number of conductors between dimmers and fixtures requires expensive multiconductor cable and multipole connectors. It has been apparent for some time that if the dimmers could be relocated at the fixtures themselves, relatively inexpensive power cable and connectors could be used to link the dimmers with the mains supply at a considerable savings in total system cost. However, the chokes required by prior art dimmer power stages would produce an unacceptable increase in weight at the fixture position.
There is, therefore, the need for a new type of dimmer power stage which requires no choke or minimizes the size, weight, and thermal losses of the inductor required.
There is also the need for such a power stage to employ an improved method of semiconductor device protection.
The specialized bulbs employed in performance lighting fixtures do not incorporate internal fusing and, on failure, can draw hundreds of amperes through the dimmer. Similarly, shorts in fixtures, wiring, or connectors can draw equally large amounts of current through the dimmer before a supply circuit breaker can open. The semiconductors employed in any professional electronic dimmer must be inherently capable of withstanding such inrush currents or be provided with the additional means to do so.
In prior art large wattage dimmers (i.e. 6000-12000 watts) the continuous currents involved require the use of thyristors whose ability to withstand inrush currents (I.sup.2 t rating) exceeds the typical fault currents encountered. Further, early concerns about thyristor quality control lead to the use of overspecified devices, whose added cost was not significant in the context of the relatively modest number of dimmers required per installation. The trend to dimmer-percircuit construction has required that new economies be achieved in the per unit production cost of dimmers in the 1000 watt and 2000 watt range. The SCRs or Triacs required by the modest continuous currents involved in such dimmers possess I.sup.2 t ratings far lower than typical fault currents (which are determined not by the wattage of the dimmer itself but by the fault current available to the dimmer from the building service as limited by the impedance presented by the supply feeder. The survivability of the dimmer's semiconductors is also affected by the quality of its choke insofar as the greater the rise time, the longer the period is available for circuit protective devices to act.) Devices with adequate I.sup.2 t can be obtained, but at a cost premium which produces an unacceptable increase in the cost of 1000 watt dimmers and an undesirable one in the case of 2000 watt units. As a result, certain modern "professional" dimmers employ semiconductors which will not survive the fault currents available in some installations. Many other dimmers in the 1000 watt range employ the combination of devices with a moderate withstand rating and a high-speed silver-sand fuse to increase the probability of survival. The requirement for a fuse, fuseholder, and circuit breaker with their associated mounting and wiring adds to both the parts and labor cost of dimmer assembly; to the front panel area requirements of each dimmer; and means the nuisance and expense of fuse replacement for the user.
It has long been apparent that a better solution would be current-limiting; sensing the fault current and shutting down the dimmer electronically before damage can occur. At least two phase-control dimmers have been built with current-limiting, but because it is impractical to turn off a thyristor before the next zero-crossing, its value for device protection is dubious. It has, therefore, also become desirable for an improved dimmer power stage to provide current-limiting as a means of device protection.
Considerable attention has been paid to the prospects for an improved dimmer power stage requiring minimal magnetics. At least three distinct approaches to such a power stage have been identified.
One such approach requiring no inductor is the "skipped half-cycle" dimmer as described in U.S. Pat. Nos. 3,691,404 and 4,287,468. This approach has, in fact, been used for the control of industrial heaters. However, at a line frequency of 60 cycles, it provides insufficient resolution for professional lighting use and can produce flickering of the bulb, particularly the low wattage bulbs typical of borderlights. While a skipped half-cycle dimmer, unlike several other approaches, can employ a conventional Triac or SCRs, such embodiments cannot provide current limiting for device protection (which is made more difficult by the lack of a rise time limiting inductor.)
A second approach to the "chokeless" dimmer is the use of high wattage power transistors operating in a pure linear mode. FIG. 2 illustrates the half-cycle waveform typical of such a dimmer at approximately half intensity. Line 201 illustrates the AC input waveform. Line 202 illustrates the output waveform.
Lacking the abrupt changes in voltage and current of the phase-control approach, the linear dimmer does not produce transients and as such EMI. Small DC versions of the linear dimmer have long been employed for the adjustment of control and instrument panel illumination. In performance lighting (where it has frequently been referred to as the "transistor" dimmer) the linear approach has long been regarded as the obvious successor to the phase control unit, awaiting only the availability of suitable devices at competitive prices.
Despite its attractive simplicity, the linear dimmer has a severe inherent limitation. In FIG. 2 shaded area 203 represents electrical energy wasted in the form of heat. As much as 25% of the load wattage must be dissipated by the devices at some settings--versus 3-5% in prior art phase control designs (only part of which is dissipated in the devices themselves). Therefore a linear dimmer must be capable of dissipating a thermal load as much as 10 times that of the devices in a phase control dimmer of identical capacity. Either massive heat sinks or some exotic form of cooling would be required; offsetting many of the benefits of choke elimination.
A third approach to a power stage with minimal magnetics is the pulse width modulated or "switched mode" dimmer.
The advantages of pulse width modulation have been well illustrated in the context of electronic power supplies. Followspots, slide and motion picture projectors, and film lighting equipment with both AC and DC gaseous discharge sources have all employed switched mode supplies to effect major reductions in weight relative to linear supplies. Switched mode dimmers, as disclosed in various U.S. Patents are employed for the intensity control of flourescent lighting fixtures. And Mole Richardson Co. of Hollywood, CA has introduced the Molelectronic.sup.r 12000 watt dimmer for the control of incandescent lamp loads operating on DC services.
The applicability of switched mode operation to the control of incandescent lamp loads operating on AC services has therefore been obvious to those skilled in the art for some time.
FIG. 3A illustrates the output half-cycle waveform typical of such a dimmer at approximately half intensity. Because the transistors operate only in a switched mode, heat rise is modest in comparison with the pure linear approach. However, the waveform produced is unacceptable for reasons of EMI generation unless filtered.
FIG. 3B illustrates the effect of adding a magnetic inductor . . . the synthesis of an amplitude-modulated sinosoidal waveform.
Although such a switched mode dimmer still requires an inductor it exploits the principle that the higher the operating frequency, the smaller the inductor. An operating frequency of 20 kHz, for example, would place the fundamental frequency beyond the range of human hearing and allow a substantial reduction in the inductor's size and weight relative to that required by a prior art phase control dimmer.
The growing volume of switched mode power supply production assures the availability of suitable control ICs, devices, and inductor materials.
The switched mode dimmer design, however, has a number of limitations. First, such dimmers would be both far more complex to design and have little commonality with prior art phase control units. The existing dimming equipment manufacturers are therefore at the lower extreme of the learning curve and development time and cost would suffer accordingly.
The switched mode dimmer is also more complex in construction, raising questions about its reliability and ease of field troubleshooting and repair relative to prior art phase control dimmers.
This relative complexity and the cost/watt projections of switched mode power supply manufacturers suggest that switched mode dimmers will be far more expensive than current phase control units. Although the cost of some components may benefit from the total volume of switched mode supply production, the switched mode dimmer itself would be sufficiently specialized that its assembly cost would not.
Nor is the switched mode dimmer's reduction in inductor size without limitation. Minimizing inductor size requires maximizing operating frequency. However as frequency increases, so do losses in the switching transistors and the inductor core, thus limiting inductor size reduction. Further, switched mode dimmers operating at higher frequencies are capable of RF interference; and as such, would require design, construction, and testing to FCC standards.
There is, therefore, a compelling demand for an improved power stage, but none yet proposed by those skilled in the art provides professional performance without either unacceptable thermal losses or complexity relative to prior art phase control units.